The ability to amplify DNA lies at the heart of modern biological and medical research. This is because most molecular biology techniques rely on samples containing many identical molecules to increase the sensitivity of an assay or to prepare enough material for further processing. Among the various nucleic acid amplification techniques, polymerase chain reaction (PCR) is the most common because of its sensitivity and efficiency at amplifying short nucleic acid sequences.
While PCR is of great utility, it is also limited in a number of ways. The first limitation of PCR is that it relies on multiple cycles of thermal melting (denaturing) at high temperatures followed by hybridization and elongation at a reduced temperature. To maximize efficiency and to minimize noise, complex temperature control of multiple reactions is required. This necessitates the use of a thermocycler controllable rapid heating/cooling block made with exotic material (e.g., gold plated silver blocks), or a robotic mechanism to move samples between temperature-controlled zones. Because of the high-temperature required to melt DNA in physiological salt conditions, PCR technology requires either the addition of fresh polymerase per cycle or the use of thermostable polymerases. The approach of adding fresh polymerase has not been automated and is thus labor intensive and prone to errors (e.g., contamination, dropped tubes, labeling errors). Furthermore, the need to add enzymes and to mix each reaction individually presents serious drawbacks that have limited adaptation of enzyme-addition PCR methods to the small scale.
Compared to methods involving the addition of fresh polymerase, the use of thermostable polymerases in PCR is the most widely practiced. This approach suffers from the fact that thermostable polymerases are found in a limited number of organisms, and the replication mechanisms used by thermophilic organisms are poorly understood. The available repertoire of thermostable polymerases is limited to single polypeptide polymerase enzymes involved in DNA repair, and/or lagging strand synthesis. DNA repair and/or lagging strand polymerases are poor choices for DNA amplification because they exhibit poor processivity (distributive synthesis). In part as a consequence of using repair and/or lagging strand polymerases (e.g. Taq, Pfu, Vent polymerases), and due to the formation of inhibitory secondary or tertiary nucleic acid structures following thermal melting, current PCR protocols do not readily amplify sequences longer than several thousands of base pairs. Reliable synthesis (and amplification) of longer templates will rely on polymerases and auxiliary enzymatic complexes collectively exhibiting much higher levels of processivity, strand displacement, and secondary structure resolution, as well as limiting the formation of inhibitory higher order nucleic acid structures that may form on cooling heat-denatured DNA.
A second limitation of PCR is that it relies on solution hybridization between oligonucleotides (PCR primers) and denatured template DNA (i.e., the DNA to be amplified) in an aqueous environment. To be effective, PCR reactions are performed in a short time because the thermostable polymerases have a rapidly declining activity at PCR temperatures. Further, for effective hybridization in a short time, a feature critical to rapid turnaround, it is necessary to perform PCR in an environment with high concentrations of oligonucleotides. The high oligonucleotide concentration also ensures rapid interaction of target sequences with the oligonucleotides in competition with the heat-denatured complementary strand still present in solution. High oligonucleotide primer concentrations can cause problems, particularly when the copy number of the target sequence is low and present in a complex mixture of DNA molecules. This would be the case, for example, in a PCR of a genome to determine the genetic polymorphism in one locus.
One problem with using high oligonucleotide concentrations is that it enhances the degree of false priming at only partly matched sequences in the complex DNA mixture. False priming refers to the hybridization of a primer to a template DNA in PCR even when the primer sequence is not completely complementary to the template nucleic acid, which can lead to non-specific amplification of nucleic acids. Noise, due to false priming, increases with the oligonucleotide concentration and the complexity of total starting DNA. In addition, the possibility of false priming increases as the copy number of target sequences decreases. Where the conditions for false priming are favorable (i.e., high oligonucleotide concentration, high complexity, low copy number), errant amplified sequences can become a dominant reaction product. Consequently it can be difficult to identify conditions, and oligonucleotides, for clean amplification of target sequences from a sample DNA without an excess of false priming background. Thus a further disadvantage of using PCR is the limited success at cleanly amplifying rare target DNAs from complex sequences mixtures.
One solution to the problems of specificity and template-melting problem incurred by PCR is to employ methods that rely on the biological properties of the bacterial RecA recombinase protein, or its prokaryotic and eukaryotic relatives. These proteins coat single-stranded DNA (ssDNA) to form filaments, which then scan double-stranded DNA (dsDNA) for regions of sequence homology. When homologous sequences are located, the nucleoprotein filament strand invades the dsDNA creating a short hybrid and a displaced strand bubble known as a D-loop. The free 3′-end of the filament strand in the D-loop can be extended by DNA polymerases to synthesize a new complementary strand. The complementary strand displaces the originally paired strand as it elongates. By utilizing pairs of oligonucleotides in a manner similar to that used in PCR it should be possible to amplify target DNA sequences in an analogous fashion but without any requirement for thermal melting (thermocycling). This has the advantage both of allowing the use of heat labile polymerases previously unusable in PCR, and increasing the fidelity and sensitivity by template scanning and strand invasion instead of hybridization.
Although the use of RecA and its homologues for in vitro amplification of nucleic acids has been previously described (U.S. Pat. No. 5,223,414 to Zarling et al., referred to herein as “Zarling”), the method and results are limited. Zarling's method has critical failings that limit its ability to achieve exponential amplification of double-stranded DNA. The failure of the Zarling method to achieve exponential amplification may be due to its specification for the use of ATPγS rather than ATP. The Zarling method urges the use of ATPγS, instead of ATP, in the assembly of RecA nucleoprotein filaments because it results in a more stable RecA/ssDNA filament structure. Normally, filaments are assembled in a 5′ to 3′ direction and will spontaneously disassemble in the same 5′ to 3′ direction as RecA hydrolyzes ATP. This process is dynamic in that assembly and disassembly occurs at the same time and the amount of assembled filaments is at equilibrium. If the non-hydrolyzable ATP analog, ATPγS, is used, hydrolysis of the ATPγS and the 5′ to 3′ disassembly of the filaments are inhibited. The great stability of RecA/ATPγS filaments, both before and after strand exchange, while helpful in the method of targeting (i.e., the Zarling method) is detrimental and unpractical for DNA amplification.
In the Zarling method, RecA protein involved in strand invasion will remain associated with the double-stranded portion of the exchanged material after strand exchange. This interaction occurs because the newly formed duplex is bound in the high-affinity site of RecA. The displaced strand occupies a different low-affinity site, unless it is bound to another single-stranded DNA binding protein (SSB), such as E. coli SSB. If ATP had been utilized to generate the exchange structure, spontaneous 5′ to 3′ disassembly might occur, although the exchange complex can be quite stable and may require additional factors to stimulate ATP-dependent disassembly. Regardless of whether spontaneous or stimulated, in the presence of ATPγS, 5′ to 3′ disassembly of the RecA filament is inhibited (Paulus, B. F. and Bryant, F. R. (1997). Biochemistry 36, 7832-8; Rosselli, W. and Stasiak, A. (1990). J Mol Biol 216, 335-52; Shan, Q. et al., (1997). J Mol Biol 265, 519-40).
These RecA/dsDNA complexes are precisely the sites targeted by the RecA/ssDNA primer complexes used to initiate subsequent rounds of invasion and synthesis. Indeed, with the RecA bound, the intermediate may not be accessible to polymerase, and certainly the dsDNAs can no longer be invaded by RecA/ssDNA primer complexes and are therefore not amplifiable from this point. Further synthesis from these templates might occur if initiated at the other end of the template, which is free of RecA, and this might eventually lead to physical displacement of the bound RecA. It is not clear, however, whether many polymerases can displace RecA in this manner. Moreover, the initiation site for that synthetic round will now be ‘blocked’ instead. In such a situation, amplification is only linear with time, and will predominately generate single-stranded DNA amplification products.
Thus, the described Zarling method, at best, is likely to generate little more than small quantities of ssDNA copies from each template. The linear amplification potentially given by the Zarling method will only occur in the presence of SSB, since the displaced strand will continue to bind to the second interaction site on RecA, and single-stranded DNA will not be released (Mazin, A. V. and Kowalczykowski, S. C. (1998). EMBO J 17, 1161-8). This probably explains why the Zarling method observed additional faster-migrating fragments when they included SSB. These additional fragments were most likely displaced single-stranded fragments. Hence, in the Zarling method only linear amplification of single-stranded DNA will occur at best. There is, therefore, a need in the art for an improved recombinase-dependent DNA amplification method.
This invention utilizes two new amplification strategies that avoid any requirement for thermal melting of DNA or thermostable components. These strategies also overcome the inefficiencies of the Zarling method. As with the Zarling strategy, these methods rely on the biological properties of the bacterial RecA protein, or its prokaryotic and eukaryotic relatives, in particular, the phage T4 uvsX protein. However, in contrast to the Zarling method, these methods are devised to achieve exponential amplification of dsDNA. They achieve this by permitting rapid regeneration of targetable sequences in the target nucleic acid in the presence of dynamic recombinase/DNA filaments. Furthermore one of the methods obviates any requirement for phased replication initiation from both ends of the target nucleic acid by coupling leading and lagging strand synthesis to simultaneously generate 2 double-stranded products.